Do you mean like a switch barrel? Or are you planning on buying two guns and just want spare parts?
Either way, unless they've changed, changing the barrel on the scandium Smiths isn't easy. As far as I know, it's a two piece system and requires some special tools to remove. And if you wanted a 4 inch, you'd need the shroud off the 329PD or 325TR and a custom barrel. All said and done, it'd be smarted to just buy more than 1 gun.